The invention concerns an optically variable element, in particular an optically variable security element for safeguarding banknotes, credit cards and the like, which has a thin film for producing color shifts by means of interference. The invention further concerns a security product and a foil, in particular an embossing foil or a laminating foil, which has such an optically variable element.
Optically variable elements are frequently used to make it difficult to copy and misuse documents or products and if possible to prevent that from happening. Optically variable elements are frequently used for safeguarding documents, banknotes, credit cards, cash cards and the like.
In order to make it difficult to copy optically variable elements, it is known for an optically variable element to be provided with a thin film layer succession which produces color shifts by means of interference, in dependence on the viewing angle.
WO 01/03945 A1 describes a security product having a transparent substrate, to one side of which is applied a thin film which produces a perceptible color shift in dependence on the change in the angle of view. The thin film comprises an absorption layer which is applied to the transparent substrate and a dielectric layer which is applied to the absorption layer. The absorption layer includes a material which is made up from one of the following materials or a combination of those materials: chromium, nickel, palladium, titanium, cobalt, iron, tungsten, molybdenum, iron oxide or carbon. The dielectric layer comprises one of the following materials or a combination of the following materials: silicon oxide, aluminum oxide, magnesium fluoride, aluminum fluoride, barium fluoride, calcium fluoride or lithium fluoride.
In order further to increase the level of safeguard against copying, a diffraction pattern is embossed on the side of the transparent substrate, which is in opposite relationship to the thin film layer succession. That diffraction pattern acts as a diffraction grating so that for example the illusion of a three-dimensional image can be produced for the viewer, by means of that two-dimensional pattern.
It is further proposed that the diffractive pattern be applied by embossing to the side of the transparent substrate to which the thin film layers are also applied.
Those two embodiments of an optically variable element provide that, at each location of the optically variable element, the optical effects produced by the thin film layers and the optical effects produced by the diffractive pattern are superimposed and this therefore overall affords an optical effect which is difficult to imitate and copy.
The invention is now based on an optically variable element as is described in WO 02/00445 A1.
The optically variable element comprises here a plurality of layers which are arranged generally in mutually superposed relationship. The optically variable element has on the one hand a thin film which produces the optical effect, already described above, of a color change which is dependent on the angle of view. In addition the optically variable element has a replication layer into which a relief structure is embossed. That relief structure produces a further optical effect, namely the diffraction effect which has already been described hereinbefore and by means of which holograms and the like can be represented. In that respect, in regard to production procedure, firstly the thin film layers are applied to the replication layer and then the relief structure is embossed thereon.
As an alternative thereto, WO 02/00445 A1 describes that the optical effect produced by the thin film structure and the optical effect produced by the relief structure are decoupled from each other. Two operating procedures are proposed for that purpose.
On the one hand it is proposed that an opaque layer is applied between the relief structure which produces a holographic image by means of diffraction and the thin film which produces a color change effect. The relief structure is screened from the thin film structure by means of that opaque layer. The second possible option involves arranging two or more layers of a substantially transparent material between the relief structure producing a holographic image by diffraction and the thin film layers. Those layers can include one or more highly refractive layers and an adhesive layer. Those layers provide for an increase in reflection and thus the strength of light in the region of the relief structure producing a holographic image.
In this respect, such a variable optical element can be produced as follows: firstly a pattern is embossed into a holographic foil. That foil is then provided in region-wise manner with a metal layer. The thin film layers are then vapor-deposited in succession. Lastly, a metal layer is applied, over the full surface area.
A further possible option involves providing a prefabricated thin film layer succession with an embossable lacquer and then embossing the relief structure into that lacquer. It is further proposed that such prefabricated thin film layers can be glued to prefabricated microstructures.
WO 02/00445 A1 thus describes either using security elements in which the optical effect produced by diffractive structures and the optical effect produced by thin film structures are coupled together, or using security elements in which the optical effect produced by diffractive structures and the optical effect produced by thin film layers are decoupled from each other.